You are here

Q: Why is CO2 stored so deep?

A: CO2 is stored deep underground because it's required by law. The US Safe Drinking Water Act orders that underground injection of any liquid, not just carbon dioxide, is below and isolated from freshwater aquifers.

Rocks are not completely solid; they are porous and liquids fill the microscopic holes. Near the surface, the liquid that fills the holes is freshwater, suitable for drinking. Deeper, it is saltwater, or brine, which is not suitable for drinking. Freshwater occurs above brine because freshwater is less dense than brine. The US Safe Drinking Water Act requires liquids to be injected into the deeper formations that contain non-potable brine.

An added technical benefit is that when we store CO2 greater than 1 km underground it takes up much less space. Like vacuum packing pillows and blankets for more efficient storage, we can fit more CO2 into the same amount of space when we store it deep underground. [See FAQ: What is a supercritical fluid?]

Want to read more?

Similar to going to outer space, the conditions deep underground in the rock formations that contain brine are very different from the conditions on Earth's surface. Those conditions have advantages for storing carbon dioxide.

The pressures deep underground are much higher. If you could go swimming one mile deep in the ocean, the weight of the water above you would squash the air in your lungs. This is called hydrostatic pressure. Underground, rock formations are saturated with water. The increased weight of water pushing down from above increases the hydrostatic pressure by 43.3 psi with each 100 feet.

It’s also hot deep underground. The center of the earth contains natural radioactive isotopes. When they undergo spontaneous normal radioactive decay processes, they generate heat. Away from tectonic margins, the temperature increases about 15 ºF with every 1,000 feet of depth.

At temperatures over 88 ºF and pressures above 1,072 psi, carbon dioxide is found in a physical state called supercritical or dense phase. In its supercritical phase, carbon dioxide is easier to inject into rock formations than if it were a liquid. It takes up about 300 times less space than if it were a gas. [See FAQ: What is a supercritical fluid?]

In the Gulf of Mexico, the physical conditions that maintain supercritical carbon dioxide occur deeper than about 1 kilometer (0.6 mile). The graphs above plot the temperature and pressure in thousands of hydrocarbon wells in the Gulf of Mexico. The colored symbols represent a subset of wells located 10 to 20 miles from the shoreline in the type of geological formations that scientists might consider initially for carbon sequestration. Note that pressures in these wells are above 1,072 psi (Pressure v Depth graph) and temperatures in these wells are over 88 ºF (Temperature v Depth graph).